According to the state of the art, automatic transmissions, particularly for motor vehicles, comprise planetary gearsets, which are shifted using friction and/or control elements, such as clutches and brakes, and which are typically connected to a starting element that is subject to a slip effect and optionally provided with a converter lockup clutch, for example a hydrodynamic torque converter or a fluid clutch.
A multi-step transmission of this type is described, for example, in DE 199 49 507 A1 by the applicant. It basically comprises a drive input shaft and a drive output shaft arranged co-axially to one another, a total of four planetary gearsets and several frictional shift elements. Two non-shiftable, front-mounted gearsets are provided on the drive input shaft, which generate two rotational speeds on the output side, which, apart from the rotational speed of the drive input shaft, can be selectively shifted to several input elements of a shiftable, multi-component main planetary gearset acting upon the drive output shaft by selectively engaging the shift elements used, and are shiftable such that, for changing from one gear to the next higher or lower gear, of the two activated shift elements, only one shift element must be engaged or disengaged. The main gearset is in turn constructed as a two-carrier, four-shaft planetary gearset, whose both planetary gearsets are firmly coupled to one another, via two elements. By using five shift elements, seven forward gears are shiftable without range shifts, and by using six shift elements even nine or ten forward gears are shiftable.
Within the scope of DE 101 15 983 A1 by the applicant, a multi-step transmission is described, with a drive input shaft that is connected to a front-mounted gearset, which is connected to a rear-mounted gearset, and to a maximum of seven shift elements, via which, by selective shifting, at least eight forward gears may be shifted into without range shifts. The front-mounted gearset is formed by a shiftable or non-shiftable planetary gearset or by a maximum of two non-shiftable planetary gearsets coupled to one another. The rear-mounted gearset is constructed as a two-carrier, four-shaft transmission with two shiftable, rear-mounted, planetary gearsets having four free shafts. The first free shaft of this two-carrier, four-shaft transmission is connected to the first control element, the second free shaft with the second and third control elements, the third free shaft with the fourth and fifth control elements, and the fourth free shaft is connected to the output shaft. According to the above named invention, connecting the third free shaft or the first free shaft of the rear-mounted gearset additionally to a sixth shift element is proposed for a multi-step transmission with a total of six shift elements. For a multi-step transmission with a total of seven control elements, the invention proposes the additional connection of the third free shaft to a sixth shift element and the first free shaft to a seventh control element.
Several other multi-step transmissions are also known, for example, from DE 101 15 995 A1 by the Applicant, where four shiftable planetary gears coupled to one another and six or seven frictional shift elements are provided, via whose selective engagement a rotational speed of a drive input shaft of the transmission is so transmitted to a drive output shaft of the transmission, that nine or eleven forward gears and at least one reverse gear can be shifted into. Depending on the transmission diagram, two or three shift elements are engaged in each gear, whereby only one shift element is disengaged at a time on shifting from one gear to each of the next higher or lower gear, and a previously disengaged shift element is engaged to avoid range shifts.
An 8-gear multi-step transmission is further known from DE 29 36 969 A1, comprising eight co-axial simple planetary gearsets and eight frictional shift elements.
A multi-step automatic transmission is further known from U.S. Pat. No. 4,683,776, comprising four simple planetary gearsets and six frictional shift elements. All four planetary gearsets are designed as so called negative planetary gearsets in simple planetary design. The first two planetary gearsets, viewed from the input drive side form a shiftable front-mounted gearset, and are firmly connected to one another, via two shaft couplings, whereby the first shaft coupling of the front-mounted gearset firmly connects both sun gears with one another and with the drive input shaft of the transmission, and whereby the second shaft coupling of the front-mounted planetary gearset connects the internal gear of the first planetary gearset and the carrier of the second planetary gearset firmly to one another, simultaneously forms the output shaft of the front-mounted gearset. The other two elements of the front-mounted gearset, i.e., the carrier of the first planetary gearset and the internal gear of the second planetary gearset, may each be attached to the transmission housing via an individual brake. The third and fourth planetary gearset form a shiftable main gearset, and are likewise connected firmly to one another via two coupling shafts, whereby the first coupling shaft of this main gearset firmly connects both sun gears of the main gearset to one another, and whereby the second coupling shaft of the main gearset firmly connects the carrier of the third planetary gearset with the gear of the fourth planetary gearset, and is likewise connectable with the drive input shaft of the transmission, and is selectively attachable to the transmission housing, via a brake. The internal gear of the third planetary gearset, as a further input element of the main gearset, is firmly connected to the output shaft of the front-mounted gearset and may be attached to the transmission housing via a further brake. The carrier of the fourth planetary gearset finally forms the output element of the main gearset and is connected firmly to the drive output shaft of the transmission. The transmission enables nine forward drive gears to be shiftable without range shifts, and one reverse gear in total, implemented by selective engagement of two of he six shift elements at a time.
Furthermore, from U.S. Pat. No. 6,176,803 B1, a 9-gear automatic transmission is known with four shiftable simple planetary gearsets arranged co-axially to one another, which is very similar in design to the automatic transmission according to U.S. Pat. No. 4,683,776. Like in U.S. Pat. No. 4,683,776, a total of six frictional shift elements are provided, two of which are implemented as multi-disk clutches and four as multi-disk brakes. Unlike U.S. Pat. No. 4,683,776, the first of the four planetary gearsets viewed from the flow of power is designed as a plus planetary gearset in double planetary design. Like in U.S. Pat. No. 4,683,776, the other three planetary gearsets are constructed as minus planetary gearsets in simple planetary design. Except for the coupling of the sun gear of the second planetary gearset with the carrier of the first (plus) planetary gearset, the coupling of the planetary gearsets, the structural coupling of the planetary gearsets among them, with both clutches and drive input shaft respectively, with the four brakes as well as with the drive output shaft are identical with U.S. Pat. No. 4,683,776. In principle, in the 9-gear automatic transmission according to U.S. Pat. No. 6,176,803 B1 only the plus planetary gearset acts as a front-mounted gearset, whereby the other three planetary gearsets form the main gearset of the transmission.
Similar multi-step automatic transmissions with nine shiftable forward gears free of range shifts are further known: from DE 42 38 025 A1, comprising four co-axial simple planetary gear sets and six or seven frictional shift elements (four multi-disk brakes and two or three multi-disk clutches). Viewed in the flow of power, the two first planetary gearsets always form a front-mounted gearset coupled with the drive input shaft of the transmission, via two firmly connected coupling shafts, where one of its planetary gearsets is implemented as a plus planetary gearset in double planetary design, and its other planetary gearset is implemented as a minus planetary design in simple planetary design. The other two planetary gearsets form a main gearset designed as a two-carrier four-shaft planetary transmission identical to U.S. Pat. No. 4,683,776.
Automatic shiftable vehicle transmissions in planetary design have in general been described in the state of the art on many occasions, and are subject to continuous development and improvements. These transmissions should feature a sufficient number of forward gears as well as one reverse gear and an excellently suitable gear ratio for motor vehicles with a high overall ratio spread as well as favorable progressive ratios. Furthermore, they should allow a high start gear ratio in the forward direction and include a direct gear, and additionally be suited for use in both passenger cars and commercial vehicles. In addition, these transmissions should require low design and construction costs, in particular a small number of shift elements, and avoid double shifts during sequential gear shifts, so that only one element at a time is changed when shifting in defined gear groups.
The object of the present invention is to propose a multi-step transmission of the type mentioned above with at least eight shiftable forward gears and at least one reverse gear, where, by using a total of four planetary gearsets, a preferably small number of shift elements is required. All forward gears should be free of range shifts during sequential gear shifts, i.e., when changing from a forward gear to the next higher or lower gear, only one previously engaged shift element should be disengaged and a previously disengaged shift element should be engaged each time. In addition, the transmission should exhibit a large transmission-ratio spread with comparably harmonic gear shifting, and manifest a favorable degree of efficiency, i.e., comparatively low drag and gear losses, in the main driving gears.